Brady can't play forever and Belicheck has been known to adjust his offense to fit the players he has (successfully, I might add). Brady will be 37 by the start of the year. He probably has 3 years or so left. Shaw can sit behind him and study one of the best QBs ever while he develops. Shaw is 22. if Brady plays 3 years he will start his career at 25. Thats a very similar career path to Rodgers. Sits behind one of the best QBs ever for three years, begins his career as a starter at 24.